The fluid-based processing of a surface film is a general system finding application in a wide range of industrial processes, including: plating and surface finishing, electrochemical transformations, deposition and polishing of materials, semiconductor device fabrication, cleaning and stripping, doping, anodizing, passivating, and so on. The number and diversity of such processes makes an exhaustive listing impractical. The factors that significantly effect the rate of such processes depend on the nature of the workpiece, the nature of the desired process, and the chemical or physical process chosen to effect such processes.
The prior art of supercritical fluid cleaning methods and systems for removing photo resist from semiconductor substrates and related requirements involves injecting fluid and additives to the cleaning chamber, and elevating the temperature and pressure to supercritical levels, where a combination of chemical and mechanical mechanisms perform the necessary work to loosen and remove the unwanted materials. The cleaning fluid mixture may be elevated in temperature and pressure to supercritical state prior to injection into the chamber, or the chamber may have internal heating elements to heat the fluid from liquid under pressure to supercritical state. Directional control of the through-flow of fluid through the chamber, as by nozzles or other flow directing devices, has provided the principle mechanical mechanism or component of the method of cleaning, directing the flow at or towards the surface at a desired angle.
An example of one known supercritical fluid cleaning process is the cleaning of a contaminating film from a silicon wafer serves to illustrate how the invention offers distinct advantages. This process typically uses a fluid to loosen, dissolve or otherwise chemically or physically transform the contaminating film on the wafer, the transformation facilitating the removal of said film. This process can conceptually be divided into several simultaneously occurring sub-processes, i.e. transport of the fluid onto or into the film, chemical or physical transformation of the film, and removal of the transformed layer to the fluid, rendering the cleaned surface. Each of these processes may be comprised of several physical or chemical elementary steps.
A well-known principle in kinetics states that the overall rate at which the process occurs is limited by the slowest sub-process or elementary step. In some cases, the slowest step may be the rate of transport of material to and from the surface of the workpiece. In such cases, the advantage of rotationally induced increased mass transport is evident. In other cases, the slowest process may be the physical removal of solvated or swelled polymer film from the surface of the workpiece, and the simple shear induced by angular acceleration acts to increase the overall process rate. In still other cases, the overall rate of the process is acceptable, but there is a question of uniformity between contiguous areas of processed surface, perhaps caused by non-uniform flow patterns inside the chamber. The time-averaging of the composition of fluid exposed to an element of surface would act to minimize undesirable effects of non-uniform chemical environment. Thus, the simple rotation of the substrate in the flowing fluid offers a combination of potentially important advantages.
A mathematical treatment of the flow characteristics of a rotating disk relate the basic advantages of the rotating workpiece design which are a consequence of the solutions of the corresponding convective diffusion equation. The details of the analysis can be found in standard textbooks—see, for example, Rotating Disk Electrode, found in Electrochemical Methods, section 8.3, page 283, by Allen J. Bard and Larry R. Faulkner, copyright 1980 John Wiley & Sons, Inc. Though not previously extended to supercritical fluid cleaning processes, this mathematical model is readily extended to describe such systems where the workpiece being cleaned is subjected to rotational movement.
According to this model, a combination of the normal and radial fluid velocities result in the acceleration of fluid in a direction normal to the surface of and toward the center of the disk, and simultaneously, a parabolic velocity profile for the fluid moving tangentially to the surface of the workpiece and outward from the center toward the edge. Thus, as fluid elements near the surface are accelerated toward the edge, a flow of fluid from the bulk solution is drawn toward the center of the disk to replace them. The relative magnitudes of all of these flow components depend upon the viscosity of the fluid, the angular velocity of the disk, and the point on the phase diagram for the fluid that corresponds to the processing conditions. The result of these actions is a combination of mass transfer to the disk and simultaneous radial shear of the film or structure at the surface, which, in the case of the removal of Photoresist, is highly advantageous.
The addition of mechanical agitation to the supercritical fluid further enhances such processes, as was discussed in this Applicant's U.S. patent application Ser. No. 10/755,432, filed Jan. 12, 2004, which is hereby incorporated by reference. The simultaneous agitation and directed mass flow of processing supercritical fluid toward a workpiece, including the associated cosolvent and dissolved chemical components, is achieved through the use of a mechanical system to rotate the workpiece during the processing. This action of rotation creates the necessary stirring of the process fluid and serves to both increase the rate of delivery of reactants to the surface of the workpiece, removal of products, and, in the case of cleaning or stripping, application of simple radial shear at the interface between the film and the surface, which increases the rate of stripping of Photoresist. These additional actions serve to enhance the overall processing, and are supplied in addition to the other advantages present in a supercritical fluid processing system, thus increasing the speed and effectiveness of such an operation. Fluid agitation, alone, however, is limited in effectiveness due to viscosity effects near the surface of the workpiece.
Even for a fluid near its critical point, the tangential component of fluid motion relative to the workpiece surface reaches a limiting value that is insufficient to efficiently remove particles of radius, or films of thickness, less than 100 nm. This is problematic since there is a well-recognized need to be able to remove material from a surface down to at least 30 nm. Rotation of the workpiece circumvents this limitation because there is no viscosity dependent upper bound on the magnitude of the centrifugal force that may be generated during a process with rotation, and is only limited by robustness of the mechanical design of the rotation mechanism itself and the compliance of the workpiece.
The rotational processing scheme also provides a unique advantage over other schemes in the application of thin film stripping. The removal of thin films, continuous or discontinuous, is a needed capability in virtually all multilayer microelectronic fabrication processes, especially related to the removal of Photoresist. As is the case with particle removal, the ever-diminishing local scale of fabricated structures is concomitant with increasing demands on processibility, with similar limitations by physical laws. In The case of thin films, however, rotation induces a mechanical strain field in the plane of the film, and this further induces incipient weaknesses in the film due to the differential acceleration of contiguous regions of the film. These actions cause stress to accumulate, and eventually lead to accelerated penetration of the film by the processing fluid and a notably enhanced rate of removal—a phenomenon similar to that commonly known as “stress cracking.” Organic polymer-based Photoresist films are especially susceptible to this mechanism of degradation, which leads to irreversible removal of even highly insoluble material.
Providing rotational capability to a wafer holder in a pressure vessel at 10,000 psi, at 150 degrees Centigrade, filled with supercritical fluid, is problematic. A conventional electric motor is unusable in this environment for many reasons including materials incompatibility, electrically conductive windings, and contamination from lubricated bearings, cooling requirements, size, electrical leads, and so forth. In addition, the fluoropolymer-based materials that might be employed to alleviate such difficulties also tend to be soluble in carbon dioxide and thus tend to create more problems than they solve. There are sealed motors, so called “can” motors that are sealed. However, cooling requirements, electrical leads, and shaft bearings and seals remain obstacles to their practical use in this application.
An external motor coupled by a rotating shaft penetrating the pressure chamber resolves some issues, but use of contact bearings on the through wall shaft and chamber-internal rotational components poses the same contamination issues, and the high working pressures make shaft seals problematic.
Magnetically coupling of an external motor to an internal shaft and rotable wafer holder has been disclosed, but again, contact bearings within a process vessel are inevitably a contamination problem to some extent.